1. Field of the Invention
The present invention relates to a line converter for a transmission line used for at least one of a microwave band and a millimeter-wave band, for example, a high-frequency module including the line converter, and a communication device.
2. Description of the Related Art
In the past, line converters for performing line conversion between a plane circuit including a dielectric substrate and a three-dimensional waveguide for propagating an electromagnetic wave in a three-dimensional space have been disclosed in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 60-192401) and Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2001-111310).
In the line converter according to Patent Document 1, an end of a micro-strip line formed as part of the plane circuit is inserted in a terminal short-circuit waveguide tube divided into two parts by a plane E of the waveguide tube. The two parts of the terminal short-circuit waveguide tube penetrate a groove formed in the dielectric substrate and sandwich the dielectric substrate therebetween.
In the line converter according to Patent Document 2, the dielectric substrate is provided at a position that is spaced away from a short-circuit plane of a terminal short-circuit waveguide tube by as much as a predetermined distance and in a predetermined direction that is perpendicular to the electromagnetic-wave propagation direction.
In the case of the line converter of Patent Document 1, there is a need to form a penetrating groove in the dielectric substrate, so as to penetrate part of the waveguide tube divided into two parts. Therefore, when the dielectric substrate is formed as a ceramic substrate including alumina or the like, it becomes difficult to machine the dielectric substrate. Further, coupling of the micro-strip line is achieved at a position where the intensity of electric fields generated by a standing wave generated at a terminal end of the waveguide is high. The coupling characteristic is determined by the positional relationship between the dielectric substrate including the micro-strip line and the waveguide tube. Therefore, the coupling characteristic is affected by the precision of assembling the dielectric substrate and the waveguide tube, which makes it difficult to obtain a line-conversion characteristic according to a predetermined design without variations.
In the line converter according to Patent Document 2, the dielectric substrate is provided in a predetermined direction that is perpendicular to the electromagnetic-wave propagation direction of the waveguide tube. Therefore, the positional relationship between the three-dimensional waveguide formed by the waveguide tube and the plane circuit formed by the dielectric substrate is determined with a low degree of flexibility. Subsequently, the plane circuit cannot be provided in a predetermined direction that is parallel to the electromagnetic-wave propagation direction of the waveguide tube.